The present invention relates to the coating of surfaces, in particular to the production of oil- and water-repellent surfaces, as well as to coated articles obtained thereby.
Oil- and water-repellent treatments for a wide variety of surfaces are in widespread use. For example, it may be desirable to impart such properties to solid surfaces, such as metal, glass, ceramics, paper, polymers etc. in order to improve preservation properties, or to prevent or inhibit soiling.
A particular substrate which requires such coatings are fabrics, in particular for outdoor clothing applications, sportswear, leisurewear and in military applications. Their treatments generally require the incorporation of a fluoropolymer into or more particularly, fixed onto the surface of the clothing fabric. The degree of oil and water repellency is a function of the number and length of fluorocarbon groups or moieties that can be fitted into the available space. The greater the concentration of such moieties, the greater the repellency of the finish.
In addition however, the polymeric compounds must be able to form durable bonds with the substrate. Oil- and water-repellent textile treatments are generally based on fluoropolymers that are applied to fabric in the form of an aqueous emulsion. The fabric remains breathable and permeable to air since the treatment simply coats the fibres with a very thin, liquid-repellent film. In order to make these finishes durable, they are sometimes co-applied with cross-linking resins that bind the fluoropolymer treatment to fibres. Whilst good levels of durability towards laundering and dry-cleaning can be achieved in this way, the cross-linking resins can seriously damage cellulosic fibres and reduce the mechanical strength of the material. Chemical methods for producing oil- and water-repellent textiles are disclosed for example in WO 97/13024 and British patent No 1,102,903 or M. Lewin et al., xe2x80x98Handbood of Fibre Science and Technologyxe2x80x99 Marcel and Dekker Inc., New York, (1984) Vol 2, Part B Chapter 2.
Plasma deposition techniques have been quite widely used for the deposition of polymeric coatings onto a range of surfaces. This technique is recognised as being a clean, dry technique that generates little waste compared to conventional wet chemical methods. Using this method, plasmas are generated from small organic molecules, which are subjected to an ionising electrical field under low pressure conditions. When this is done in the presence of a substrate, the ions, radicals and excited molecules of the compound in the plasma polymerise in the gas phase and react with a growing polymer film on the substrate. Conventional polymer synthesis tends to produce structures containing repeat units which bear a strong resemblance to the monomer species, whereas a polymer network generated using a plasma can be extremely complex.
The success or otherwise of plasma polymerisation depends upon a number of factors, including the nature of the organic compound. Reactive oxygen containing compounds such as maleic anhydride, has previously been subjected to plasma polymerisation (Chem. Mater. Vol. 8, 1, 1996).
U.S. Pat. No. 5,328,576 describes the treatment of fabric or paper surfaces to impart liquid repellent properties by subjecting the surfaces to a pre-treatment with an oxygen plasma, followed by plasma polymerisation of methane.
However, plasma polymerisation of the desirable oil and water repellent fluorocarbons have proved more difficult to achieve. It has been reported that cyclic fluorocarbons undergo plasma polymerisation more readily than their acyclic counterparts (H. Yasuda et al., J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2411). The plasma polymerization of trifluoromethyl-substituted perfluorocyclohexane monomers has been reported (A. M. Hynes et al., Macromolecules, 1996, 29, 18-21).
A process in which textiles are subjected to plasma discharge in the presence of an inert gas and subsequently exposed to an F-containing acrylic monomer is described in SU-1158-634. A similar process for the deposition of a fluroalkyl acrylate resists on a solid substrate is described in European Patent Application No. 0049884.
Japanese application no. 816773 describes the plasma polymerisation of compounds including fluorosubstituted acrylates. In that process, a mixture of the fluorosubstituted acrylate compounds and an inert gas are subjected to a glow discharge.
The applicants have found an improved method of producing polymer and particular halopolymer coatings which are water and/or oil repellent on surfaces.
According to the present invention there is provided a method of coating a surface with a polymer layer, which method comprises exposing said surface to a plasma comprising a monomeric unsaturated organic compound which comprises an optionally substituted hydrocarbon group, wherein the optional substituents are halogen; provided that where the compound is a straight chain perhalogenated alkene, it includes at least 5 carbon atoms; so as to form an oil or water repellent coating on said substrate.
Unsaturated organic compounds are those which contain at least one double bond which is capable of reacting to form a polymeric compound. The compounds used in the method of the invention suitably include at least one optionally substituted hydrocarbon chain. Suitable chains, which may be straight or branched, have from 3 to 20 carbon atoms, more suitably from 6 to 12 carbon atoms
Monomeric compounds used in the method may include the double bond within a chain and so comprise alkenyl compounds. Alternatively, the compounds may comprise an alkyl chain, optionally substituted by halogen, as a substitutent which is attached to an unsaturated moiety either directly or by way of an functional group, such as a ester or sulphonamide group.
As used therein the term xe2x80x9chaloxe2x80x9d or xe2x80x9chalogenxe2x80x9d refers to fluorine, chlorine, bromine and iodine. Particularly preferred halo groups are fluoro. The term hydrocarbon includes to alkyl, alkenyl or aryl groups. The term xe2x80x9carylxe2x80x9d refers to aromatic cyclic groups such as phenyl or napthyl, in particular phenyl. The term xe2x80x9calkylxe2x80x9d refers to straight or branched chains of carbon atoms, suitably of up to 20 carbon atoms in length. The term xe2x80x9calkenylxe2x80x9d refers to straight or branched unsaturated chains suitably having from 2 to 20 carbon atoms.
Monomeric compounds where the chains comprise unsubstituted alkyl or alkenyl groups are suitable for producing coatings which are water repellent. By substituting at least some of the hydrogen atoms in these chains with at least some halogen atoms, oil repellency may also be conferred by the coating.
Thus in a preferred aspect, the monomeric compounds include haloalkyl moieties or comprise haloalkenyls. Therefore, preferably the plasma used in the method of the invention will comprise a monomeric unsaturated haloalkyl containing organic compound.
Suitable plasmas for use in the method of the invention include non-equilibrium plasmas such as those generated by radiofrequencies (Rf), microwaves or direct current (DC). They may operate at atmospheric or sub-atmospheric pressures as are known in the art.
The plasma may comprise the monomeric compound alone, in the absence of other gases or in mixture with for example an inert gas. Plasmas consisting of monomeric compound alone may be achieved as illustrated hereinafter, by first evacuating the reactor vessel as far as possible, and then purging the reactor vessel with the organic compound for a period sufficient to ensure that the vessel is substantially free of other gases.
Particularly suitable monomeric organic compounds are those of formula (I) 
where R1, R2 and R3 are independently selected from hydrogen, alkyl, haloalkyl or aryl optionally substituted by halo; and R4 is a group Xxe2x80x94R5 where R5 is an alkyl or haloalkyl group and X is a bond; a group of formula xe2x80x94C(O)O(CH2)nYxe2x80x94 where n is an integer of from 1 to 10 and Y is a bond or a sulphonamide group; or a group xe2x80x94(O)pR6(O)q(CH2)txe2x80x94 where R6 is aryl optionally substituted by halo, p is 0 or 1, q is 0 or 1 and t is 0 or an integer of from 1 to 10, provided that where q is 1, t is other than 0.
Suitable haloalkyl groups for R1, R2, R3 and R5 are fluoroalkyl groups. The alkyl chains may be straight or branched and may include cyclic moieties.
For R5, the alkyl chains suitably comprise 2 or more carbon atoms, suitably from 2-20 carbon atoms and preferably from 6 to 12 carbon atoms.
For R1, R2 and R3, alkyl chains are generally preferred to have from 1 to 6 carbon atoms.
Preferably R5 is a haloalkyl, and more preferably a perhaloalkyl group, particularly a perfluoroalkyl group of formula CmF2m+1 where m is an integer of 1 or more, suitably from 1-20, and preferably from 6-12 such as 8 or 10.
Suitable alkyl groups for R1, R2 and R3 have from 1 to 6 carbon atoms.
Preferably however, at least one of R1, R2 and R3 is hydrogen and preferably R1, R2, R3 are all hydrogen.
Where X is a group xe2x80x94C(O)O(CH2)nYxe2x80x94, n is an integer which provides a suitable spacer group. In particular, n is from 1 to 5, preferably about 2.
Suitable sulphonamide groups for Y include those of formula xe2x80x94N(R7)SO2 where R7 is hydrogen or alkyl such as C1-4alkyl, in particular methyl or ethyl.
In a preferred embodiment, the compound of formula (I) is a compound of formula (II)
CH2xe2x95x90CHxe2x80x94R5xe2x80x83xe2x80x83(II)
where R5 is as defined above in relation to formula (I).
In compounds of formula (II), X in formula (I) is a bond.
In an alternative preferred embodiment, the compound of formula (I) is an acrylate of formula (III)
CH2xe2x95x90CR7C(O)O(CH2)nR5xe2x80x83xe2x80x83(III)
where n and R5 as defined above in relation to formula (I) and R7 is hydrogen or C1-6 alkyl, such as methyl.
Using these compounds, coatings with water hydrophobicity values of up to 10 and oleophobicity values of up to 8 have been achieved as illustrated hereinafter.
Other compounds of formula (I) are styrene derivatives as are well known in the polymer art.
All compounds of formula (I) are either known compounds or they can be prepared from known compounds using conventional methods.
The surface coated in accordance with the invention may be of any solid substrate, such as fabric, metal, glass, ceramics, paper or polymers. In particular, the surface comprises a fabric substrate such as a cellulosic fabric, to which oil- and/or water-repellency is to be applied. Alternatatively, the fabric may be a synthetic fabric such as an acrylic/nylon fabric.
The fabric may be untreated or it may have been subjected to earlier treatments. For example, it has been found that treatment in accordance with the invention can enhance the water repellency and confer a good oil-repellent finish onto fabric which already has a silicone finish which is water repellent only.
Precise conditions under which the plasma polymerization takes place in an effective manner will vary depending upon factors such as the nature of the polymer, the substrate etc. and will be determined using routine methods and/or the techniques illustrated hereinafter. In general however, polymerisation is suitably effected using vapours of compounds of formula (I) at pressures of from 0.01 to 10 mbar, suitably at about 0.2 mbar.
A glow discharge is then ignited by applying a high frequency voltage, for example at 13.56 MHz.
The applied fields are suitably of average power of up to 50 W. Suitable conditions include pulsed or continuous fields, but are preferably pulsed fields. The pulses are applied in a sequence which yields very low average powers, for example of less than 10 W and preferably of less than 1 W. Examples of such sequences are those in which the power is on for 20 xcexcs and off for from 10000 xcexcs to 20000 xcexcs.
The fields are suitably applied for a period sufficient to give the desired coating. In general, this will be from 30 seconds to 20 minutes, preferably from 2 to 15 minutes, depending upon the nature of the compound of formula (I) and the substrate etc.
Plasma polymerisation of compounds of formula (I), particularly at low average powers has been found to result in the deposition of highly fluorinated coatings which exhibit super-hydrophobicity. In addition, a high level of structural retention of the compound of formula (I) occurs in the coating layer, which may be attributed to the direct polymerisation of the alkene monomer for instance a fluoroalkene monomer via its highly susceptible double bond.
It has been noted, particularly in the case of the polymerisation of compounds of formula (III) above, that low power pulsed plasma polymerisation produces well-adhered coatings which exhibit excellent water and oil repellency. The greater level of structural retention in the case of pulsed plasma polymerisation can be attributed to free radical polymerisation occurring during the duty cycle off-time and less fragmentation during the on-time.
In a particularly preferred embodiment of the invention, a surface is exposing a surface to a plasma comprising a compound of formula (III) as defined above, wherein the plasma being created by a pulsed voltage also as described above.
Suitably the compound of formula (I) includes a perfluoroalkylated tail or moiety, the process of the invention may have oleophobic as well as hydrophobic surface properties.
Thus the invention further provides a hydrophobic or oleophobic substrate which comprises a substrate comprising a coating of a alkyl polymer and particularly a haloalkyl polymer which has been applied by the method described above. In particular, the substrates are fabrics but they may be solid materials such as biomedical devices.